Methane clathrates are common constituents of the shallow marine geosphere, and they occur both in deep sedimentary structures, and as outcrops on the ocean floor. Methane hydrates are believed to form by migration of gas from depth along geological faults, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water. Methane clathrates are also present in deep Antarctic ice cores, and record a history of atmospheric methane concentrations, dating to 800,000 years ago.[5] The ice-core methane clathrate record is a primary source of data for global warming research, along with oxygen and carbon dioxide.